1. Field of the Invention
The present invention relates to a signal processing unit for use in electronic equipment and, specifically, relates to a signal processing unit for use in a radio telecommunication apparatus such as a pager, a cordless telephone, or a mobile telephone.
2. Description of the Prior Art
In recent years, a new type of a paging system has been developed in which a base station transmits message information, and any called one of the pager receivers subscribed to this system displays the message information. The paging signal used in this system is schematically shown in FIG. 9. As is illustrated in this figure, each frame of the paging signal is time-divided into n groups. Further, each group is time-divided into one synchronization or preamble word and m calling words. Each calling word is formed of an identification (ID) code and a message code.
To call a selected pager receiver, a caller dials a telephone numbers assigned to the pager receiver, thus reaching the base station. Then, the caller sends message information to the base station. The base station generates the ID code assigned to the selected pager receiver and the message code corresponding to the message information, and inserts these codes into one of vacant calling words of that group of the paging signal to which the selected pager receiver belongs. Then, the base station transmits the paging signal.
As is illustrated in FIG. 10, each pager receiver comprises an antenna 1, a receiver circuit 10, a control circuit 4, and ID ROM 5, a drive circuit 6, a loudspeaker 7, an LCD (Liquid-Crystal Display) 8, and a clock generator 9,. The receiver circuit 10 has a radio circuit 2 and a demodulation circuit 3. In operation, the antenna 1 catches the paging signal which the base station has transmitted. The signal is supplied to the receiver circuit 10. In the circuit 10, the radio circuit 2 receives the paging signal, and the demodulation circuit 3 demodulates the paging signal. The demodulated signal is output from the receiver circuit 10 to the control circuit 4.
The control circuit 4 has, for example, a microcomputer (CPU). The CPU is driven by a clock signal having a frequency of, for example, 480 KHz, generated from the clock generator 9. The control circuit 4 collates the ID code inserted in the demodulated paging signal, with the ID code stored in the ID ROM 5 and assigned to the pager receiver. If the ID codes are identical, the control circuit 4 determines that the pager receiver has been called. Then, the control circuit 4 supplies a drive signal to the drive circuit 6. The circuit 6 drives the loudspeaker 7, and the loudspeaker 7 generates a beep sound, telling the user of the pager receiver that his receiver is being called. When the ID codes are found to be identical, the message code is supplied from the receiver circuit 10 to the control circuit 4. The circuit 10 corrects error bits, if any, of the message code, and then demodulates the message code into the original message information. The control circuit 4 outputs this message information to the LCD 8, which displays the message information.
Generally, a method known as "battery saving technique" is adopted in each pager receiver to save power so as to lengthen the lifetime of the battery loaded in the pager receiver. More precisely, the control circuit 4 controls the power-source unit including the battery such that power is supplied from the battery to the receiver circuit 10 only while the antenna 1 is receiving group signals which is transmitted for a group of pagers including the pager. No power is supplied to the circuit 10 while any other groups are transmitted. Since n groups form the frame, the power consumed in the circuit 10 is about 1/n of the power which is required if power is supplied to the circuit 10 all the time. Even under the battery saving technique, control circuit 4 is continuously powered and driven by a clock signal.
In a prior art pager, a frequency of the clock signal is set to be so high that error correction operations are properly performed against the received message signal accompanied by an error connection code. The high frequency art which the control circuit 4 is continuously driven causes considerable power consumption. This is because the power is consumed greater in a microprocessor device constituting the control circuit 4 as the frequency at which the microprocessor device is driven increases.